1. Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly to a rate-responsive pacemaker having an automatic sensor threshold with a programmable offset.
In Applicants' earlier U.S. Pat. No. 4,940,052, there is disclosed a microprocessor-controlled, rate-responsive pacemaker having automatic rate response threshold adjustment. The present invention relates to an improvement over the invention disclosed in the '052 patent. The '052 patent is incorporated herein by reference.
As taught in the '052 patent, the rate-responsive pacemaker therein disclosed includes a physiological sensor that generates a raw signal, or "raw sensor signal," as a function of a sensed physiological parameter, which physiological parameter provides an indication of what the pacing rate of the pacemaker should be. The raw sensor signal is converted to one of a plurality of discrete sensor level index signals. The particular sensor level index signal derived from the raw sensor is then used, in conjunction with a selected Slope parameter, to point to a particular sensor-indicated rate (SIR) signal. The SIR signal may then be used by the pacemaker to define a pacing rate.
In order to prevent inappropriate increases in pacing rate while a patient is at rest or at low levels of activity, the '052 patent offers a plurality of programmable sensor rate response threshold values. The selected sensor response threshold value represents a minimum level of patient activity that must occur before the raw sensor signal is considered sufficiently large to represent meaningful physiological activity that should increase the SIR signal. One embodiment disclosed in the '052 patent provides for the automatic setting of the sensor rate response threshold by averaging the sensor index signal over a prescribed period to time and by adding thereto a fixed threshold offset value. In a preferred embodiment, the '052 patent teaches that the sensor level index signals be averaged over a preceding 18 hour period, and that a fixed offset of two (2) be added. (Note, that as used herein the sensor "offset" is measured in sensor units, which are relative units related to the maximum raw sensor output signal. For example, the full scale sensor output may be represented by 13 sensor units.)
Unfortunately, while the "autothreshold" feature of the invention disclosed in the '052 patent represents a valuable tool for a physician to use in programming the parameters of a rate-responsive pacemaker, the fixed sensor offset that is added to the 18 hour average as taught in the '052 patent makes such autothreshold feature impractical and undesirable for most patients. This is because a fixed offset, even when added to an automatically adjustable threshold, does not invoke the same pacemaker behavior in different patients. A sensor threshold and offset that provides ideal performance in one patient may be over response or under-responsive in another patient, or even for the same patient at a future time. Thus, each patient is unique, and requires a highly customized and versatile setting of the sensor rate response threshold and offset. What is needed, therefore, is a more versatile way of conditioning the raw sensor signal to fit the needs of a particular patient over time, and a way of programmably and/or automatically setting the "offset" that is used in conjunction with determining a sensor threshold.
The present invention advantageously addresses the above and other needs.